The present invention relates to methods and systems for operating battery packs, and more particularly, to operating battery packs that are deep cycled in electric vehicle applications, in a manner that enhances performance, longevity and safety.
The way that a battery pack is operated in the field can significantly affect its performance, longevity and safety. This concern is particularly relevant for demanding applications, such as use in electric vehicles.
By way of example, consider a battery pack that is formed from a number of lithium cells connected together in series. The lifetime of the lithium battery pack degrades dramatically if the voltage across one of its cells falls below a predetermined threshold during discharge (typically three volts), or rises above a predetermined threshold during charging (typically 4.2 volts). For this reason it is very important that cell voltages be carefully monitored and measures be taken to maintain cell voltages in a particular range.
To complicate matters further, manufacturing defects in lithium cells result in some cells that do not hold as much charge as other seemingly identical cells. For this reason, when a number of lithium cells are connected together in series, defective cells discharge quicker than the other cells and thus are the first to hit the lower threshold described above during discharge. Such defective cells are often also the first to hit the upper threshold during charging. This imbalance between cells limits the effective range of operation of the battery pack, unless the charge is rebalanced during operation of the battery pack.